1. Field of the Invention
The present invention relates to a process and system for treating crude oil vapors such as those produced from crude oil loading or any hydrocarbon vapor comprised of light hydrocarbons in the range of C2-C3, i.e. ethane and propane and derivative hydrocarbons, and heavy hydrocarbons in the range of C4-C6 and greater, i.e. butane, butane derivative hydrocarbons, and heavier hydrocarbons.
2. Description of the Related Art
When loading crude oil or other hydrocarbons containing light hydrocarbons in the range of C2-C3 and heavy hydrocarbons in the range of C4-C6 and greater onto a tank truck or tanker vessel, some of the light and heavy hydrocarbons vaporize. These vapors may be initially treated to remove such things as sulfur and those vapors that will condense to a liquid when cooled are recovered as a liquid. From that point, the remaining vapor which contains a large amount of light and heavy hydrocarbons must be treated before venting.
For safety and environmental reasons, venting of hydrocarbons is often accomplished through a flare or other combustion device which burns the hydrocarbons. However, burning of hydrocarbons produces carbon dioxide, i.e. a greenhouse gas, and other pollutants. Thus the practice of burning the hydrocarbons through a flare or other combustion device is environmentally undesirable. Also, burning of the hydrocarbons wastes valuable resources which might otherwise be recovered for use as fuel or as a salable product.
Currently hydrocarbon vapor recovery systems consist of passing the vapors through an activated carbon bed. Activated carbon attracts hydrocarbon material on its surface, with a higher preference for adsorbing the heavy hydrocarbons in the range of C4-C6 and greater. Thus, the activated carbon bed will selectively adsorb most of the heavy hydrocarbons in the range of C4-C6 and greater. The light hydrocarbons in the range of C2-C3 will generally pass through the activated carbon bed which has already adsorbed on it heavy hydrocarbons, and those light hydrocarbons will be vented to atmosphere or be vented to a flare to be burned.
Once the carbon bed is loaded with hydrocarbons during the adsorption phase, it is then taken off line and regenerated by subjecting it to a vacuum. The hydrocarbons that were adsorbed onto the carbon bed will be drawn off by the vacuum and the discharge from the vacuum pump will next be transferred to the inlet of a liquid contact absorption unit. Within the liquid contact absorption unit, the vapor will pass through a liquid hydrocarbon shower, such as for example gasoline, where the gaseous hydrocarbons will be absorbed in the liquid hydrocarbons, thereby increasing the amount of liquid hydrocarbons exiting the unit. This recovers much of the heavy hydrocarbons in the range of C4-C6 and greater.
However, even though this type of treatment does recover much of the heavy hydrocarbons in the range of C4-C6 and greater, most of the light hydrocarbons in the range of C2-C3 pass through the carbon bed without being adsorbed and are either vented to atmosphere or are vented to a flare where they are burned. Both options provide no benefit or profit to the operation and both venting and flaring create environmentally undesirable situations.
The present invention addresses this problem by providing a two stage carbon bed adsorption system consisting of a first carbon bed adsorption unit and a second carbon bed adsorption unit in series with the first one. The first stage carbon bed adsorption unit removes the heavy hydrocarbons in the range of C4-C6 and greater, and the vapor discharged from the first stage carbon bed adsorption unit is then passed through a second stage carbon bed adsorption unit where the light hydrocarbons in the range of C2-C3 are removed from the remaining gaseous components of the vapor before the vapor is then discharged to atmosphere. The second stage vapor contains minimal heavy hydrocarbons which improves the working capacity for the light hydrocarbons.
During regeneration of the first stage carbon bed, the heavy hydrocarbons in the range of C4-C6 and greater are drawn off in the discharge from a vacuum pump and are subjected to further treatment to liquefy them so that they can be recovered as a liquid product for storage.
During regeneration of the second stage carbon bed, the light hydrocarbons in the range of C2-C3 are drawn off in the discharge from a second vacuum pump as a gaseous vapor containing a rich concentration of hydrocarbons which can be used as fuel gas for facility operations.